The painted turtle, chrysemys picta, possesses several unique features of its gastroentero-pancreatic endocrine system: (1) it exhibits a diffuse endocrine pancreas and (2) the intestine is characterized by the presence of insulin-containing endocrine cells in the mucosal epithelium. I. The painted turtle's diffuse endocrine pancreas will be characterized as a model for the investigation of B cell function in an animal which lacks the compact, closely-ordered islet characteristic of mammals and many other vertebrates. Insulin secretion in response to glucose and to other insulinotropic agents will be assessed in several defferent test systems employing a heterologous radioimmunoassay. Pancreatic B cell response in vivo will be determined using a glucose tolerance test. In vitro B cell sensitivity to known secretagogues (glucose, arginine, tolbutamide, carbamycholine) and the dynamics of insulin release from pancreatic pieces will be characterized in a perifusion system. The effects of the pancreatic hormones glucagon and somatostatin on insulin release will be assessed in the perifusion system while the effects of antisera to these islet hormones will be assessed in an in situ perfusion system. Results from these latter experiments will provide evidence as to the presence or absence of paracrine control of endocrine secretion in an animal which lacks a compact well-organized islet. II. The second aspect of this study will determine the distribution of the intestinal insulin cells and assess their susceptibility to alloxan, a beta-cytotoxic drug as measured by a decrease in immunocytochemically stained cells containing insulin and by a decrease in radioimmunoassayable insulin in mucosal scrapings. Insulin release from intestinal insulin cells will be assessed by perfusion of 10 cm segments of intestine challenged with known insulin secretagogues such as glucose or arginine. In summary, the absence of closely ordered pancreatic islets in Chrysemys picta may be advantageous as model for studying pancreatic endocrine secretion in a disorganized endocrine pancreas such as occurs in animal and human diabetes where alteration of islet cell topography is often quite marked. Additionally, investigation of intestinal insulin cell secretory function may provide insight as to the prevertebrate function of insulin given the evolutionary origin of the B cells in the intestine.